I have a new to me 67 XKE 4.2 9:1 running after sitting 30 years. It has run well on several short trips to test its reliability. Started it for another trip and heared bad noises, shut it off. Put camera in spark plug hole and found a broken ring part on top of piston. Tore down engine, had 4 pistons with broken rings, 2 of which had broken ring parts that made it’s way to the top of piston. So I will need .040" over sized piston to fix the bores. With good gas being harder to find, am i better off installing 8:1 pistons?
I certainly would: you won’t lose enough performance to really being that noticeable, and you’ll be able to use less expensive fuel.
Really???
That aside, 8:1 are perfectly fine. I‘d stay with the engine number/ original ratio. Are your bores that bad?
That usually requires a big chunk of the piston above the land to break free.
All my XK engines have 8:1 pistons, which give you more options at the pump. My first MK1 came to me with .040" over 12:1 Mahle racing pistons, which limited my choices to Sunoco 260 or American Super Premium. I can’t say I ever had any day-to-day use for any extra power that may have been created.
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In the case of a four stroke engine, yes: I was wondering about that myself. Happens all the time with two strokes, but that’s a different intake flow path.
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Yep: the only mechanism I’ve seen it occur with on a 4-stroker is a burned/broken crown.
Not to say that it can’t happen without that, but I can’t imagine how.
They look like you’re already running 8:1 pistons?
https://forums.jag-lovers.com/t/height-of-domed-pistons/359468
Hmmm…skirts very long (and split) on one and very short on the other( delete joke) margins and crowns?. I’m no expert so will step aside for those better qualified…
7:1 and 8:1 pistons, hopefully not from the same engine?
Different vintage, the ones in my picture are from the 80s but the crowns and geometry are the same.
IIRC, technically, 4.2 ‘9:1’ pistons are 8.7:1 ratio, in the later XJ6 engines, at least.
Well yeah… technically 
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